Compounding and Mixing
Compounding is to combine the appropriate blend of polymers, fillers, oils and pigments for a specific compound formula in a closed mixer in batches of 180 kg (400 lbs) to 500 kg (1100 lbs). Batch temperatures are closely controlled, as are mixing power, cycle time and rotor speed, in accordance with the compound specification
Mixing is the process of applying mechanical work to the ingredients in order to blend them into a homogeneous substance. Internal mixers are often equipped with two counter-rotating rotors in a large housing that shear the rubber charge along with the additives. The mixing is done in three or four stages to incorporate the ingredients in the desired order. The shearing action generates considerable heat, so both rotors and housing are water-cooled to maintain a temperature low enough to assure that vulcanization does not begin.
After mixing, the rubber charge is dropped into a chute and fed by an extruding screw into a roller die. Alternatively, the batch can be dropped onto an open rubber mill system. A mill consists of twin counter-rotating rolls which are responsible for producing a thick rubber sheet and warming up compounds prior to extrusion or calendering. The sheet is pulled off the rollers in the form of a strip. The strip is cooled, dusted with talc, and laid down into a pallet bin.
The ideal compound at this point would have a highly uniform material dispersion; however in practice there is considerable non-uniformity to the dispersion. This is due to several causes, including hot and cold spots in the mixer housing and rotors, excessive rotor clearance, rotor wear, and poorly circulating flow paths. As a result, there can be a little more carbon black here, and a little less there, along with a few clumps of carbon black elsewhere, that are not well mixed with the rubber or the additives.